A phablet is a gadget that does the job of a smartphone and a tablet at once: big enough to read and watch things on, still a phone you hold and call with. The word is a squeeze of phone and tablet. It had a moment in the 2010s, when huge phones first felt new and a bit odd. Though actually, most phones sold now are already this size, so the word has faded. People often just say 'a big phone', or they name the model. It is informal, magazine-and-shop English, not a science term, and not a brand.